The present invention relates to modulators, and more particularly, to switched-inverter modulators for use in driving magnetrons and microwave tubes, and the like.
Most high energy pulse power devices require some type of high voltage modulation scheme to drive them. The conventional techniques used are L-C line-type, hard tube or semi-hard tube modulators. The present invention was developed to minimize system size and weight and maximize power efficiency while incorporating waveform agility into the modulator design. Consequently, what was required was the efficiency of a line-type modulator, pulse width and PRF (pulse repetition frequency) versatility of a hard tube modulator, and a third very important parameter, the ability to provide time varying amplitude that is not provided by the conventional modulators.
There are three types of modulators that are conventionally used to drive high power microwave tubes, for example, which include line-type, hard-tube (or semi-hard tube), and modulating anode/grid modulators. The present invention does not employ modulating anodes or grids and these types of modulators will therefore not be discussed herein.
Line-type modulators are very efficient and use voltage doubling resonance effects of an L-C network driving an impedance matched load such as a microwave tube by way of a pulse transformer. L-C networks are PRF limited due to the slow recovery time of a thyratron. Also, pulse widths are generally limited to the microsecond region due to the enormous size and weight of a PFN (pulse forming network) used in line-type modulators. The line-type modulator, when driving a radio frequency (RF) magnetron for example, is not particularly amplitude variable pulse-to-pulse and therefore was an unlikely candidate for the application.
Hard-tube modulators are pulse width and PRF versatile, but again have their limitations. Hard tubes require more heating and biasing power than the line-type modulator. Hard tubes are lossy and are considered temperamental. Although higher PRFs than a line-type modulator can usually be achieved, the switch tubes tend to be pulse width limited, typically in the short microsecond region. In typical capacitively coupled hard tube modulators, amplitude modulation is not easily achieved. Issues of droop verses amplitude variation become a concern.
Semi-hard-tube modulators (modified hard-tube design) incorporate a device known as a Crossatron used in place of a vacuum tube. The Crossatron is more efficient and allows pulse widths to be extended to the millisecond region. However, this modulation scheme lacks versatile amplitude modulation capability. The waveform of the semi-hard-tube modulator has longer pulses in the millisecond region in addition to the narrow pulse widths. The PRF is also varied while maintaining constant amplitude pulses.
Therefore, it is an objective of the present invention to provide for an improved switched-inverter modulator for use in driving magnetrons and microwave tubes, and the like, that overcome the limitations of conventional modulators.